CMC and carbon-carbon composite materials are used in various fields where their thermostructural properties, i.e. their very good mechanical properties, make it possible to build structural elements that are heavily stressed, and that have the ability to retain these mechanical properties even at relatively high temperatures. This applies, for example, in the field of space, in particular for panels providing thermal protection or nozzles for thrusters, in the field of aviation, e.g. for parts of airplane jets, and in the field of friction, in particular for airplane brake disks.
Chemical vapor infiltration of a material into a porous substrate consists in placing the substrate inside an enclosure, in causing a gas to diffuse within the accessible internal pores of the substrate, which gas contains at least a precursor of the material in the gaseous state, and simultaneously in controlling in particular the temperature and the pressure inside the enclosure so that a deposit is formed from the precursor throughout the volume of the substrate. The precursor for carbon can be an alkane, an alkyl, or an alkene, giving rise to pyrolytic carbon by decomposition. For chemical vapor infiltration of a ceramic material, a gas is diffused that contains one or more gaseous species giving the desired ceramic material by decomposition or by mutual chemical reaction. Thus, for example, chemical vapor infiltration of silicon carbide (SiC) can be obtained by means of a gas containing methyltrichlorosilane (MTS) and in the presence of hydrogen gas (H.sub.2). Gaseous species that are precursors for other ceramics such as carbides, nitrides, or oxides are well known to the person skilled in the art.
Several vapor infiltration methods exist, in particular isobaric methods that are isothermal and isobaric methods that have a temperature gradient.
In isothermal isobaric methods, the substrates to be densified are maintained at all times at uniform temperature throughout their volume and under uniform pressure. A drawback thereof lies in the impossibility, in practice, of obtaining densification that is uniform. The matrix material tends to deposit preferentially within pores close to the outside surface of the substrate. The progressive obstruction of the surface pores makes access for the gas to the inside of the material more and more difficult, resulting in a gradient of densification between the surface and the core of the material. It is of course possible to descale or machine the surface of the substrate one or more times during the densification process in order to open up surface pores. However that requires the process to be interrupted for the time required to extract the substrate from the densification installation, to cool it down, to descale it, to put the substrate back into the installation, and to return to the desired temperature.
With a temperature gradient type method, the above-mentioned drawback of the isothermal method can be limited to a great extent. A temperature difference is established between a hotter inner portion and a cooler surface of the substrate which is exposed to the gas. The matrix material then deposits preferentially in the hotter inner portion. By controlling the temperature of the substrate surface so that it is below the decomposition or reaction threshold of the gas, at least during an initial portion of the densification process, it is possible to ensure that the densification front progresses from the inside towards the surface of the substrate as the process continues. In conventional manner, the temperature gradient can be obtained by placing one or more substrates around a susceptor coupled to an inductor with an inside face of the substrate(s) in contact with the susceptor. It is also possible to obtain a temperature gradient by direct inductive coupling with the substrate being densified, when the nature of the substrate makes that possible. Those techniques are described in particular in the documents FR-A-2 711 647 and U.S. Pat. No. 5,348,774. In the latter document, the substrates are heated both by coupling with a susceptor and by direct coupling with the substrates as the densification front progresses. Means are provided to measure variation in the weight of the substrates continuously so as to monitor the progress of the densification process. As a function of the measured weight variation, the process can be optimized, in particular concerning its duration, by acting on densification parameters, and in particular on the power supplied to the inductor. Monitoring variation in substrate weight also makes it possible to define the end of the densification process. The temperature gradient method does indeed make it possible to obtain densification that is more uniform than with the isothermal process, but it can only be implemented with substrates of a particular shape, in particular substrates that are annular.
Whatever the densification method used, the microstructure of the material deposited within the substrate depends on the conditions under which chemical vapor infiltration takes place. For example, with pyrolytic carbon, by modifying these infiltration conditions, it is possible, in particular, to obtain pyrolytic carbon of the smooth laminar type, of the dark laminar type, of the rough laminar type, or of the isotrophic type. The microstructure of pyrolytic carbon is a characteristic that is important with respect to the properties of the densified substrate. Thus, for parts made of carbon-carbon composite, it is often desirable to have a microstructure of the rough laminar type, in particular because of its ability to be graphited by heat treatment. Controlling the microstructure of the material deposited within the substrate is also important for a material of the ceramic type.
In isothermal densification methods, it has been observed that in spite of the infiltration parameters being fixed initially so as to give a deposit having the desired microstructure, the microstructure can vary during the densification process. The difficultly of conserving uniform microstructure is to be observed in particular when densifying thick substrates such as fiber preforms that are more than 5 cm thick.
The same difficulty exists with temperature gradient densification methods, whether by inductive coupling with a susceptor in contact with the substrates or by inductive coupling directly with the substrates.